In moisture-responsive elements which change in electrical resistance or electrical capacity in response to ambient moisture or vapor, the following materials have been used:
(1) Sintered metal oxide membranes such as iron oxides (Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4), tin oxides (SnO.sub.2) and the like; PA1 (2) Electrolytes such as lithium chloride (LiCl); PA1 (3) Dispersions of electrically conductive particles or fibers in hygroscopic resin or polymer membrane; PA1 (4) Temperature-sensing elements such as thermistors; and PA1 (5) hydrophilic polymer membranes or polymeric electrolytes.
In general, the moisture responsive elements consisting of the metal oxides suffer from drawbacks, for example, a high electrical resistance which is considerably dependent on temperature, despite their excellent heat resistance and rapid moisture-responsive property. Particularly, the moisture responsive property of sintered metal oxides is not easily reproducible, and such elements are not easily interchanged, because the moisture responsive property is dependent on factors such as particle size, density of the sintered metal oxides and the like.
The moisture responsive elements consisting of electrolytes such as lithium chloride have only a narrow range of operation, so that two or more different kinds of moisture responsive elements are required in order to detect relative humidity over a broad range of, for example, from 0 to 100 percent, i.e. the whole humidity range. Further, there is a problem related to the useful life time of the electrolyte moisture responsive element. When held under a high moisture condition, i.e under a relative humidity of more than 90 percent, the electrolyte is leached or diluted and the moisture responsive property is considerably deteriorated.
The moisture responsive elements made by dispersing electrically conductive particles or fibers in moisture absorbable resin are sensitive and change their electrical resistances in high moisture circumstances, but are not sensitive in low moisture circumstances. Therefore, such moisture responsive elements are not suitable to detect humidity over a wide range and are mainly used for detection of dropwise condensation.
The moisture responsive elements using temperature-sensing elements such as thermistors detect ambient humidity in response to the change of heat conductivity of gas or air in concert with the humidity therein. Therefore, these elements can be used to determine an absolute humidity, but are liable to be affected by ambient temperature and wind.
Moisture responsive elements with hydrophilic polymer membranes or electrolyte polymer membranes have been known. These are responsive to moisture over a wide range with sensitivity. Further, as the element has a simple structure and is comparatively easily produced, the cost for production can be minimized. However, such elements using polymer membranes have poor moisture or water resistance and a short life time, which is a very serious problem.
The problems in the moisture responsive elements as immediately aforementioned are caused by deficient water or moisture resistance of the moisture responsive membrane. That is, a conventional hydrophilic polymer membrane or electrolyte polymer membrane is dissolved or deteriorated, though slowly, under a high humidity condition i.e. a relative humidity of more than 90 percent, so as to lose its moisture responsive property. Particularly, when dewdrops are condensed on the surface of the moisture responsive membrane or the membrane is soaked in water, the membrane is rapidly dissolved to substantially lose its moisture responsive property.